Page 110 - Chemical equilibria Volume 4

86 Chemical Equilibria
We can see from the figure that, in standard conditions, zinc oxide cannot
be reduced by carbon monoxide, because the Ellingham line for
equilibrium [3R.23] is situated, at 700°C, below that relative to equilibrium
[3R.24].
The straight line given by equation [3.52] intersects the straight line relative
to equilibrium [3R.23] at point A, according to relation [3.51]. Thus, the
intersection of this straight line with a horizontal placed above the diagram or a
vertical placed on the side if the diagram defines a scale P CO2/P CO, graduated
-8
10
from 10 to 10 . Thus, extending line CA enables us to directly read the value
of that ratio at equilibrium of reaction [3R.24] at 700°C.
Similarly, for the reduction of oxides by hydrogen, the diagram contains:
– a line corresponding to the oxidation of hydrogen, which begins from a
0
point H with the coordinates (-273°C, Δ g 0 (0àK = Δ h = 480 kJ);
)
r r
– a scale of the ratios of pressures P H2O/P H2.
3.3.2.9. Complete Ellingham diagram for oxide–metal systems

Having chosen to write all the reactions with a gaseous molecule on the
left-hand side, we can plot the Ellingham lines for the different reactions on
the same graph.

Figure 3.12 shows plots all of the oxides diagram on one graph. Here are
see the graduation of the standard Gibbs energies on the left-hand vertical
axis, the point of origin Ω, and on the right-hand vertical axis, the
logarithmic scale in oxygen pressure. The figure also shows points C and H.
In order to make it easier to plot, we have not shown the graduated axes in
P HO / P and P CO 2 / P .
CO
H
2
2
We can easily use the diagram to find slope ruptures due to changes of
the physical state, which are identified, by the letters F, B, S and T (fusion,
boiling, sublimation and polymorphic transformation respectively).

Also note the straight line of oxidation of the carbon in carbon monoxide,
which has a negative slope because of the increase in the number of gaseous
molecules during the course of the reaction, which gives highly-positive
entropy. Similar diagrams exist for nitrides and carbides [DES 10].